1. Field of the Invention
The present invention relates to a soldering machine, and more particularly to a soldering machine suitable for use with lead-free solders.
2. Description of the Related Art
In conventional soldering machines, heat sources are employed for heating solder up to temperatures higher than the melting point of the solder. Known examples of such a heat source include an infrared heater for directly heating a target to be heated as disclosed in JP,A 6-253465 and JP,A 10-335047, for example, and a fluid that is heated by an infrared heater and blown toward a target to be heated, as disclosed in JP,A 11-54903 and JP,A 9-186448, for example.
From the standpoint of suppressing environmental pollution, it has been recently tried to employ, as solders for use in soldering machines, lead-free solders containing no lead instead of the conventional solder containing lead. As lead-free solders, there are known tin-silver (SnAg) solder, tin-copper (SnCu) solder, tin-silver (SnAg) solder added with bismuth (Bi), and so on. These lead-free solders have the molting points higher than that of the conventional lead solder. For example, the melting point of the SnAg solder is 221xc2x0 C., the melting point of the SnCu solder is 227xc2x0 C., and the melting point of the SnAg solder added with Bi is 205xc2x0 C. Thus, the melting points of the lead-free solders are in the range of 200 to 230xc2x0 C. higher than that of the conventional lead solder.
On the other hand, electronic parts soldered onto a printed circuit board include chip parts such as SOP (Small Outline Package) type ICs, QFP (Quad Flat Package) type ICs, resistors and capacitors, as well as electrolytic capacitors. Some of those soldered parts have the endurable temperatures near the melting points of the lead-free solders. The endurable temperature of an aluminum electrolytic capacitor, for example, is 250xc2x0 C. Therefore, in the case of soldering the aluminum electrolytic capacitor with a solder having the melting point of 227xc2x0 C., the solder is required to be heated up to a level of, e.g., 230xc2x0 C. higher than the melting point of the solder. On the other hand, taking into account the endurable temperature of the electronic part to be mounted on a printed circuit board, the heating temperature is required to be kept not higher than 240xc2x0 C. Thus, the heating temperature must be controlled so as to fall within the range of 230 to 240xc2x0 C.
However, when an infrared heater is used to directly heat a target to be heated as disclosed in the above-cited JP,A 6-253465 and JP,A 10-335047, there occurs a difference in temperature of the heated target under an influence of the configuration of the infrared heater. Accordingly, heating solder up to a level higher than the melting point of the solder may result in that electronic parts to be mounted on a printed circuit board are partly heated beyond their endurable temperatures. In such an event, the electronic parts are thermally damaged. Also, when a fluid heated by an infrared heater is blown toward a target to be heated as disclosed in the above-cited JP,A 11-54903 and JP,A 9-186448, a variation in temperature distribution of the heated target is smaller than that resulting when an infrared heater is used to directly heat a target to be heated. It is however difficult to realize uniform heating over the entirety of a printed circuit board and electronic parts to be mounted thereon, which are objects subjected to the heating, so that the heating temperature is held within the range of 230 to 240xc2x0 C. Such a difficulty has accompanied a problem that the electronic parts may be thermally damaged.
Accordingly, it is an object of the present invention to provide a soldering machine using lead-free solders, which can uniformly heat a printed circuit board and electronic parts to be mounted thereon, and which can solder the electronic parts without thermally damaging them.
(1) To achieve the above object, the present invention provides a soldering machine including a heating furnace unit in which a fluid supplied from a blower is heated by a heater and the heated fluid is blown to a heating target to heat the heating target, wherein the heating furnace unit comprises a porous member disposed between the blower and the heater for making uniform pressure of the fluid supplied from the blower, the porous member having a number of holes formed therein to allow the fluid to flow through the porous member toward the heater; and a radiation plate disposed between the heater and the heating target for blowing the fluid having been heated by the heater to the heating target in the form of a turbulent flow.
In the thus-constructed soldering machine using lead-free solders, a printed circuit board and electronic parts to be mounted thereon can be uniformly heated, and the electronic parts can be soldered without thermally damaging them.
(2) In above (1), preferably, the heating furnace unit further comprises an absorber provided on a surface of at least one of the radiation plate and the heater on the side facing the heating target, the absorber absorbing those of infrared rays emitted from the heater which have wavelengths not longer than 1 xcexcm and not shorter than 20 xcexcm.
With that construction, a far infrared ray with the wavelength of 1 xcexcm to 20 xcexcm is irradiated to the heating target. Therefore, the temperatures of ICs, etc. even in the form of black molded packages can be easily controlled so that the package temperatures are kept below the endurable level.
(3) In above (1), preferably, a plurality of the heating furnace units are connected in the direction of feed of the heating target, and a stream of the fluid is formed to flow in a direction opposite to the direction of feed of the heating target.
With that construction, a lowering of the temperature in the soldering section can be prevented.
(4) In above (1), preferably, the soldering machine further includes a cooling unit for cooling the heating target having been heated by the heating furnace unit, the cooling unit comprising a blower for blowing a cooling fluid to the heating target; a cooler for cooling the fluid of which temperature has increased after cooling the heating target; and a porous member disposed between the blower and the heating target for making uniform pressure of the fluid supplied from the blower, the porous member having a number of holes formed therein to allow the fluid to flow through the porous member toward the heating target.
With that construction, since the heating target is subjected to quick cooling after being heated, it is possible to produce a finer structure inside the solder, suppress growth of an acicular Ag3Sn crystal within the solder, and prevent a soldering failure that conductors and electronic parts of the heating target are peeled off from the soldered points.
(5) In above (1), preferably, a plurality of the cooling units are connected in the direction of feed of the heating target, and a stream of the fluid is formed to flow in a direction opposite to the direction of feed of the heating target.
With that construction, since a stream of the cooling fluid is formed to flow in a direction opposite to the direction of the heating target, consumption of inert gas used as the cooling fluid can be reduced.